The present invention relates to a method for die-casting of concrete goods such as block stones, which are cast in a cellular casting mould by this being completely filled with concrete, whereafter a superjacent holder-on or dolly provided with mutually flush load plates having a shape corresponding to the respective subjacent cell shapes is pressed down against the concrete surface for compression of the casting material in the individual cells, preferably during vibration of the casting material from the underside of the mould. After the required compression, the dolly is lifted from the mould, and the moulded stone bodies are demoulded.
For the supply of concrete to the mould cells, a so-called filler cart is normally used, which with its bottom open is deplaced from a position in which it is placed on a fixed bottom plate underneath the outlet for casting concrete from a silo, whereby it will bring a charge of concrete over the mould and during its return movement scrape off the excess amount of concrete for which there has been no room in the cells. Thus, the mould is left with a smooth concrete surface level with the upper edges of the partitions between the cells.
Thereafter, the downwards pressing of the dolly is effected, e.g. by hydraulic power, the dolly being moved parallelly for obtaining that all the stones are compressed to the same height. Because of variations of the processing conditions and of the character of the casting concrete, the height of the stones will not always be the same, but as a height variation of .+-.2 mm is normally accepted, the result will almost always be usable anyway.
The stones must meet the requirement that their strength must be as good as possible, and that there must be only a small variation of the strength of the different stones of the production. As a certain stone strength variation inevitably occurs, the problem occurs in practice that for ensuring a desired minimum strength, it is necessary to use so much cement in the casting concrete that the weakest stones have the concerned minimum strength, whereby the other stones within the scope of variation will have an unnecessarily high cement content, i.e. generally the production requires, in reality, an unnecessarily high consumption of cement, which is a considerable disadvantage, economically as well as resource-wise.
The invention, which aims at fighting this disadvantage, is based on the consideration that the strength of the stone bodies is proportional to their density, and that thus, the strength variation will primarily be a question of density variations in the achieved result of the compression. By effecting that the density variations are equalized, the cement may thus be dosed in a better optimized manner for obtaining a cement economy in the running production, which justifies even a rather costly modification of the mould equipment.
The said stone strength variation is primarily caused by the fact that by the used simple method of casting concrete supply, an entirely uniform or compact filling of the cells is not obtained, as, locally, there may be more or less air in the concrete, and as this air only escapes by the compression and the vibration of the concrete, it is not possible in advance to effect any individual adjustment for obtaining a uniform result.
The invention is furthermore based on the observation that the normally occurring variation in the weight quantity of concrete of a uniform and good quality filled into the cells amounts to appx. 4%, which by an individually impressed compression pressure of a normal magnitude gives a resulting height difference of appx. 3 mm for typical block stones. When the products are compressed to the same height, the best filled cells will correspondingly give products having the highest density and thus the best strength. What is noticable here, however, is that the said height difference between products which have been individually compressed and thereby compressed to more or less the same density and strength is smaller than the pre-accepted height difference tolerance. This means that by controlling the production fairly closely with respect to concrete quality and processing parameters, it is necessary to use only a small part of the thickness tolerance for those variations which are caused by these conditions, while the rest of the tolerance interval may be used for obtaining a uniform density and strength by effecting an individual compression of the concrete in the different cells, whereby it is possible to continue using the said, very simple concrete supply method.
Thus, by the invention care is taken that the load plates of the dolly are pressed against the concrete in the individual cells with primarily the same force, but in a mutually independent manner with regards to movement. This may be obtained by each load plate being placed individually on an operation cylinder, these cylinders being carried on a common dolly base and supplied with pressure from the same pressure source, but as in practice only relatively small thickness variations for the stone members are concerned, it is fully sufficient to use load plates which are connected to the common dolly base through resiliently compressible means. These may easily be shaped such that for the resulting pressure force it will be of no special importance whether the load plate penetrates a few millimeters more or less down into the mould cells.
In the following, the invention, which also comprises the concerned moulding equipment, is described in more detail with reference to the drawings.